Flat panel display

ABSTRACT

A flat panel display capable of reducing element defects by decreasing taper angles of contact holes and a via hole. The flat panel display includes a thin film transistor having at least source and drain electrodes formed over an insulating substrate, an insulating layer having a via hole for exposing one of the source and drain electrodes, and an anode connected to said one of the source and drain electrodes through the via hole. The via hole and the anode are tapered with taper angles of 60° or less. The source and drain electrodes are connected respectively to source and drain regions of the thin film transistor through the contact holes. The contact holes are also tapered with taper angles of 60° or less.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 10/938,080 filed Sep. 10, 2004, now U.S. Pat. No. 7,656,087,which claims priority to and the benefit of Korean Patent ApplicationNo. 2003-84786, filed Nov. 27, 2003, the disclosure of each of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flat panel display and, moreparticularly, to a full color active matrix organic light emittingdisplay (AMOLED), which lowers taper angles of an anode and a via holeto prevent defects from occurring.

2. Description of the Related Art

In general, AMOLED has a plurality of pixels arranged on a substrate ina matrix format, and each pixel includes an electroluminescent (EL)element with an anode, an organic emission layer, and a cathode that arelayered together. The AMOLED also includes a thin film transistor (TFT)connected to the EL element as an active element for driving the ELelement.

FIG. 1 is a cross-sectional view of a bottom-emitting organic lightemitting display (OLED) in accordance with the prior art. Referring toFIG. 1, a buffer layer 105 is formed on an insulating substrate 100, anda semiconductor layer 110 having source and drain regions 111 and 115 isformed on the buffer layer 105. A gate electrode 125 is formed on a gateinsulating layer 120, and source and drain electrodes 141 and 145 areformed on an interlayer insulating layer 130, and are connected to thesource and drain regions 111 and 115 through contact holes 131 and 135,respectively. This way, the TFT is fabricated.

An anode 170 as a lower electrode is formed on a passivation layer 150,and is connected through a via hole 155 to the drain electrode 145 ofthe source and drain electrodes 141 and 145. An organic emission layer185 and a cathode 190 as an upper electrode are formed on the substrate.In this manner, the organic EL element is fabricated.

In the conventional organic light emitting display having theabove-mentioned structure, when taper angles θ11 and θ12 of the contactholes or via hole are large, pin hole defects have occurred at steppedportions of the anode 170 and near the contact hole or via hole, or opencircuit defects between the anode and the cathode have occurred. Inaddition, the organic emission layer has not been deposited on thestepped portions of the anode and near the contact hole and via hole, orit has not been uniformly deposited to thereby have a thickness thinnerthan those of other portions. Thus, when a high voltage is appliedbetween the anode and the cathode, the portion where the organicemission layer is not deposited or thinly deposited has current densityconcentrated to generate circular dark spots. Due to the occurrence ofthe dark spots, emission regions become reduced to thereby deteriorateimage quality.

In addition, the cathode to be deposited over the entire surface of thesubstrate is not densely formed in the stepped portion, which causesexternal oxygen or moisture to penetrate through the portion that is notdensely formed. Thus, when a high voltage is applied between the anodeand the cathode, the portion that is not densely deposited has a void inthe cathode due to electromigration resulting from the current densityconcentration, so that high heat occurs due to a resistance increaseresulting from external oxygen penetration. As a result, the portion hascircular dark spots generated as time proceeds.

FIG. 4 is a cross-sectional photograph near a contact hole forindicating the deterioration mechanism resulting from a high taper angleof the contact hole or via hole in the conventional OLED. Referring toFIG. 4, oxygen or moisture penetrates through the open portion of thecathode 190 resulting from pin hole defects near the contact hole or viahole, so that the deterioration may be diffused.

FIG. 5 is a graph for explaining a relationship between the number ofdefects and taper angles of a contact hole or via hole in theconventional OLED. Referring to FIG. 5, it can be seen that the defectsnear the contact hole or via hole may be prevented when the taper angleof the contact hole or via hole is 60° or less. FIG. 6 is a photographthat shows occurrences of dark spots at the edge portion of the emissionregion when the taper angle of the contact hole or via hole is large.Referring to FIG. 6, it can be seen that a lot of dark spots occur nearthe edge portion of the emission region when the taper angle of thecontact hole or via hole is 75°. In this case, the reference numerals 61and 62 indicate dark spots occurred near via hole and the contact hole,respectively.

U.S. Pat. No. 5,684,365 discloses a technique, which limits a taperangle of a passivation layer at an edge of an opening for exposing someportions of an anode. FIG. 2 is a cross-sectional view of theconventional bottom-emitting OLED. Referring to FIG. 2, a buffer layer205 is formed on an insulating substrate 200, and a semiconductor layer210 having source and drain regions 211 and 215 is formed on the bufferlayer 205. A gate electrode 225 is formed on a gate insulating layer220, and source and drain electrodes 241 and 245 are formed on aninterlayer insulating layer 230, and are connected to the source anddrain regions 211 and 215 through contact holes 231 and 235,respectively. Further, an anode 270 as a lower electrode is formed onthe interlayer insulating layer 230, and is connected to the drainelectrode 245.

A passivation layer 250 formed of an insulating layer such as a siliconnitride layer is deposited to have a 0.5 to 1.0 μm in thickness on thesubstrate, and is etched to form an opening 275 for exposing someportions of the anode 270. In this case, the passivation layer 250 isarranged at the edge of the opening 275 to have a taper angle θ22 of 10°to 30° with respect to the anode 270. An organic emission layer 285 anda cathode 290 as an upper electrode are then formed on the substrate.

In the conventional flat panel display having the above-mentionedstructure, the taper angle θ22 of the passivation layer contacted withthe anode is limited to be in a range of 10° to 30° when the passivationis etched to expose some portions of the anode so as to prevent defectsof the organic emission layer from occurring. However, pin hole or opencircuit defects still occur near the contact hole, the via hole, or inthe stepped portion as shown in FIG. 4 and FIG. 5, and dark spots arealso generated due to the cathode that is not densely deposited.

In addition, U.S. Pat. No. 6,246,179 discloses a technique employing anorganic insulating layer with planarizing property to prevent defectsfrom occurring near the via hole, contact hole, or in the steppedportion. FIG. 3 is a cross-sectional view of the OLED having theconventional top-emitting structure. Referring to FIG. 3, a buffer layer305 is formed on an insulating layer 300, and a semiconductor layer 310having source and drain regions 311 and 315 is formed on the bufferlayer 305. A gate electrode 325 is formed on a gate insulating layer320, and source and drain electrodes 341 and 345 are formed on theinterlayer insulating layer 330, and are connected to the source anddrain regions 311 and 315 through contact holes 331 and 335,respectively.

A planarizing layer 360 is formed on a passivation 350, and an anode 370as a lower electrode is formed on the planarizing layer 360, and isconnected through a via hole 355 to one of the source and drainelectrodes 341 and 345, for example, to the drain electrode 345. A pixeldefining layer 365 is then formed to have an opening 375 for exposingsome portions of the anode 370, and an organic emission layer 385 and acathode 390 as an upper electrode are formed on the anode 370 and thepixel defining layer 365.

In the above-mentioned OLED, a taper angle θ31 of the pixel defininglayer is limited in a range of 20° to 80° to prevent defects of theorganic emission layer, and the planarizing layer is used for preventingelement defects from occurring near the contact hole or via hole due tothe stepped portion of the substrate surface. However, reliability ofthe element is dependent on the taper angle between the pixel defininglayer and the anode. By way of example, the organic emission layer andthe cathode are likely to be deteriorated at the edge of the openingwhen the taper angle is large, and there is a limit to reducing thetaper angle and the thickness of the pixel defining layer because of aparasitic capacitance and a step resulting from wiring when the taperangle is small.

Further, an aperture ratio is further decreased with the increasing useof the pixel defining layer, and outgas from the pixel defining layercauses the emission region to be reduced, which leads to pixel sizereduction, so that lifetime and image quality are deteriorated, andadditional processes for depositing and etching the pixel defining layerare required.

SUMMARY OF THE INVENTION

In an exemplary embodiment according to the present invention, a flatpanel display capable of preventing defects from occurring by decreasinga taper angle at an edge of an anode, is provided.

In another exemplary embodiment according to the present invention, aflat panel display capable of preventing defects from occurring bydecreasing taper angles of a contact hole and a via hole, is provided.

In yet another exemplary embodiment according to the present invention,a flat panel display capable of increasing an aperture ratio, isprovided.

In yet another exemplary embodiment according to the present invention,a flat panel display capable of lengthening the lifetime and increasingthe image quality, is provided.

According to an aspect of the present invention, there is provided aflat panel display, which includes an insulating substrate, a lowerconductive layer formed on the insulating substrate, an upper conductivelayer formed above the lower conductive layer, and an insulating layerformed between the upper conductive layer and the lower conductivelayer. The insulating layer has an interconnecting hole used forinterconnecting the upper conductive layer and the lower conductivelayer. The interconnecting hole is tapered with a taper angle of 60° orless with respect to a surface of the insulating substrate.

The interconnecting hole may have a taper angle in a range of 14° to45°. The lower conductive layer may include source and drain regions ofa thin film transistor, the upper conductive layer may includesource/drain electrodes, and the interconnecting hole may includecontact holes used for connecting the source and drain regions to thesource and drain electrodes, respectively. The lower conductive layermay be one of a source electrode and a drain electrode of a thin filmtransistor, the upper conductive layer may be an anode, and theinterconnecting hole may be a via hole used for connecting said one ofthe source electrode and the drain electrode of the thin film transistorto the anode.

According to another aspect of the present invention, there is provideda thin film transistor, which includes a semiconductor layer having asource region and a drain region formed on an insulating substrate. Agate electrode is formed above the semiconductor layer, a sourceelectrode is connected to the source region of the semiconductor layer,and a drain electrode is connected to the drain region of thesemiconductor layer. A gate insulating layer is formed between thesemiconductor layer and the gate electrode, and an interlayer insulatinglayer is formed between the source and drain electrodes and the gateelectrode. The gate insulating layer and the interlayer insulating layerhave contact holes for connecting the source and drain regions to thesource and drain electrodes, respectively. Each said contact hole istapered with a taper angle of 60° or less with respect to a surface ofthe insulating substrate.

The taper angle may be 45° or less. A minimum value of the taper angleof each said contact hole may be determined by the equation θ=tan⁻¹(d1/d2), wherein d1 is a horizontal distance, from a position on one ofthe source and drain electrodes where a distance between said one of thesource and drain electrodes and the gate electrode is minimum, to abottom edge of a corresponding said contact hole, and d2 is a sum ofthicknesses of the gate insulating layer and the interlayer insulatinglayer at the position on said one of the source and drain electrodeswhere the distance between said one of the source and drain electrodesand the gate electrode is minimum.

According to yet another aspect of the present invention, there isprovided a flat panel display, which includes a thin film transistorhaving at least source and drain electrodes formed on an insulatingsubstrate, an insulating layer having a via hole for exposing one of thesource and drain electrodes, and an anode connected to said one of thesource and drain electrodes through the via hole. The via hole istapered with a taper angle of 60° or less with respect to a surface ofthe insulating substrate, and the anode is tapered with a taper angle of60° or less with respect to the surface of the insulating substrate.

The taper angle of the via hole may be 45° or less, and the taper angleof the anode may be 45° or less. A minimum value of the taper angle ofthe anode may be determined by the equation θ=tan⁻¹ (d1/d2), wherein d1is the thickness of the anode, and wherein d2 is a difference between alength of a lower surface of the anode and length of an upper surface ofthe anode.

According to yet another aspect of the present invention, there isprovided a flat panel display, which includes a lower electrode formedon an insulating substrate, an organic emission layer formed on thelower electrode, and an upper electrode formed on the organic emissionlayer. The lower electrode is tapered with a taper angle of 60° or lesswith respect to a surface of the insulating substrate, and a minimumvalue of the taper angle of the lower electrode is determined by theequation θ=tan⁻¹ (d1/d2), wherein d1 is a thickness of the lowerelectrode, and d2 is a difference between a length a lower surface ofthe lower electrode and a length of an upper surface of the lowerelectrode.

The lower electrode may be one of an anode and a cathode, and the upperelectrode may be the other one of the anode and the cathode.Alternatively, the lower electrode may be a transmitting electrode, andthe upper electrode may be a reflective electrode, and light emittedfrom the organic emission layer may be directed toward the substrate.Alternatively, the lower electrode may be a reflective electrode, andthe upper electrode may be a transmitting electrode, and light emittedfrom the organic emission layer may be directed in a direction away fromthe substrate. Alternatively, the lower electrode and the upperelectrode may be transmitting electrodes, and light emitted from theorganic emission layer may be directed both toward the substrate and inan opposite direction away from the substrate.

The organic emission layer may include at least one thin layer selectedfrom a hole injecting layer, a hole transporting layer, an emissionlayer, a hole blocking layer, an electron transporting layer, and anelectron injecting layer. The organic emission layer may be formed by alaser induced thermal imaging method, an inkjet printing method, or adeposition method.

According to yet another aspect of the present invention, there isprovided a flat panel display, which includes an insulating substrate,and a semiconductor layer having source and drain regions of a thin filmtransistor formed on the insulating substrate, and a first insulatinglayer having contact holes for exposing a portion of the source regionand a portion of the drain region. Source and drain electrodes areconnected respectively to the source and drain regions through thecontact holes. A second insulating layer is formed on the firstinsulating layer and has a via hole for exposing one of the source anddrain electrodes. A lower electrode is formed on the second insulatinglayer and is connected to said one of the source and drain electrodes ofthe thin film transistor through the via hole. An organic emission layeris formed on the lower electrode, and an upper electrode is formed onthe organic emission layer. The via hole is tapered with a taper angleof 60° or less with respect to the surface of the insulating substrate,and the lower electrode is tapered with a taper angle of 60° or lesswith respect to the surface of the insulating substrate.

According to yet another aspect of the present invention, there isprovided a flat panel display, which includes an insulating substrate, asemiconductor layer having source and drain regions of a thin filmtransistor formed on the insulating substrate, and a gate electrodeformed over the semiconductor layer. A first insulating layer is formedbetween the semiconductor layer and the gate electrode. A secondinsulating layer has contact holes for exposing a portion of the sourceregion and a portion of the drain region, and source and drainelectrodes are connected respectively to the source and drain regionsthrough the contact holes. A third insulating layer formed on the secondinsulating layer has a via hole for exposing one of the source and drainelectrodes. A lower electrode is formed on the third insulating layerand is connected to said one of the source and drain electrodes of thethin film transistor through the via hole. An organic emission layer isformed on the lower electrode, and an upper electrode is formed on theorganic emission layer. The via hole and the contact holes are taperedwith taper angles of 60° or less with respect to a surface of theinsulating substrate, and the lower electrode is tapered with a taperangle of 60° or less with respect to the surface of the insulatingsubstrate.

According to yet another aspect of the present invention, there isprovided a flat panel display, which includes an insulating substrate, asemiconductor layer having source and drain regions of a thin filmtransistor formed on the insulating substrate, and a gate electrodeformed above the semiconductor layer. A first insulating layer is formedbetween the semiconductor layer and the gate electrode. A secondinsulating layer has contact holes for exposing a portion of the sourceregion and a portion of the drain region. Source and drain electrodesare connected respectively to the source and drain regions through thecontact holes. A lower electrode is formed on the second insulatinglayer and connected to one of the source and drain electrodes. A thirdinsulating layer for passivation has an opening for exposing a portionof the lower electrode. An organic emission layer is formed on the lowerelectrode and the third insulating layer, and an upper electrode isformed on the organic emission layer. The contact holes and the lowerelectrode are tapered with taper angles of 60° or less with respect to asurface of the insulating substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will become moreapparent to those of ordinary skill in the art by describing in detailcertain exemplary embodiments thereof with reference to the attacheddrawings in which:

FIG. 1 is a cross-sectional view of an organic light emitting device(OLED) in accordance with the prior art;

FIG. 2 is a cross-sectional view of an OLED having a tapered passivationlayer in accordance with the prior art;

FIG. 3 is a cross-sectional view of an OLED having a pixel isolationlayer in accordance with the prior art;

FIG. 4 is a photograph showing occurrences of defects due to an opencircuit of a cathode in a conventional OLED;

FIG. 5 is a graph illustrating a relationship between the number ofdefects and taper angles of a contact hole or via hole in a conventionalOLED;

FIG. 6 is a photograph which illustrates dark spots near a contact holeand a via hole among emission regions in a conventional OLED;

FIG. 7 is a cross-sectional view of an OLED in accordance with a firstexemplary embodiment of the present invention;

FIG. 8A explains the principle how the minimum taper angle for a contacthole is determined in an OLED in accordance with the first exemplaryembodiment of the present invention;

FIG. 8B explains the principle how the minimum taper angle for an anodeis determined in an OLED in accordance with the first exemplaryembodiment of the present invention;

FIG. 9 is a cross-sectional view of an OLED in accordance with a secondexemplary embodiment of the present invention;

FIG. 10 is a cross-sectional view of an OLED in accordance with a thirdexemplary embodiment of the present invention;

FIG. 11A and FIG. 11B are cross-sectional views which illustrateprocesses for a first method for fabricating an organic light emittingdisplay in accordance with the third exemplary embodiment of the presentinvention;

FIG. 12A and FIG. 12B are cross-sectional views which illustrateprocesses for a second method for fabricating an organic light emittingdisplay in accordance with the third exemplary embodiment of the presentinvention;

FIG. 13A and FIG. 13B are cross-sectional views which illustrateprocesses for a third method for fabricating an organic light emittingdisplay in accordance with the third exemplary embodiment of the presentinvention; and

FIG. 14 is a photograph of a normal emission region when the taper angleof a contact hole or via hole is 60° or less in an OLED in accordancewith an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which certain exemplaryembodiments of the present invention are shown. This invention may,however, be embodied in different forms and should not be construed asbeing limited to the embodiments set forth herein. In the drawings, thethickness of layers and regions are exaggerated for clarity. Likereference numerals/characters designate like elements throughout thespecification and the drawings.

Referring to FIG. 7, a buffer layer 405 is formed on an insulatingsubstrate 400, and a semiconductor layer 410 having source and drainregions 411 and 415 is formed on the buffer layer 405. A gate electrode425 is formed on the gate insulating layer 420, and source and drainelectrodes 441 and 445 are formed on an interlayer insulating layer 430,and are connected to the source and drain regions 411 and 415 throughcontact holes 431 and 435, respectively.

In this and other exemplary embodiments, one or more layers and/orelements may be referred to as formed on or being on the insulatingsubstrate even though there may be intervening layers/elements betweenthe insulating substrate and said layers and/or elements. By way ofexample, The semiconductor layer 410 may be referred to as being formedon the insulating substrate 400 even though the buffer layer 405 isdisposed therebetween.

An anode 470 as a lower electrode is formed on a passivation layer 450,and is connected through a via hole 455 to one of the source and drainelectrodes 441 and 445, for example, to the drain electrode 445. Afterthe anode 470 is formed, an organic emission layer 485 and a cathode 490are sequentially formed. The organic emission layer 485 includes atleast one thin layer selected from a hole injecting layer, a holetransporting layer, an R, G, or B emission layer, a hole blocking layer,an electron transporting layer, and an electron injecting layer.

To prevent element defects in the contact hole and via hole fromoccurring, in the first exemplary embodiment of the present invention,the interlayer insulating layer 430 and the gate insulating layer 420are etched to form the contact holes 431 and 435 having a first taperangle θ41, and the passivation layer 450 is etched to form the via hole455 having a second taper angle θ42.

The first taper angle θ41 of the contact holes 431 and 435 refers to anangle formed by sides of the contact holes 431 and 435 with respect tothe substrate surface, and the second taper angle θ42 refers to an angleformed by the side of the via hole 455 with respect to the substratesurface. The contact holes 431 and 435 and the via hole 455 should havethe first and second taper angles θ41 and θ42 of 60° or less,respectively, so as to prevent the defects from occurring as shown inFIG. 5. Preferably, the contact holes 431 and 435 and the via hole 455should have taper angles of 45° or less.

The contact holes 431 and 435 are used to connect upper conductivelayers to lower conductive layers. By way of example, the contact holes431 and 435 are used respectively to connect the source and drainelectrodes 441 and 445 to the source and drain regions 411 and 415formed on the semiconductor layer 410, and the source and drainelectrodes 441 and 445 are formed adjacent to the gate electrode 425.Thus, the gate electrode 425 and the source and drain electrodes 441 and445 should be arranged to be spaced from each other by a predetermineddistance in order to minimize the interference between the gateelectrode 425 and the source and drain electrodes 441 and 445. Thus, thefirst taper angle θ41 should maintain an angle not less than apredetermined value.

FIG. 8A shows the contact hole 435 for connecting the drain electrode445 to the drain region 415 among the contact holes 431 and 435 of FIG.7.

Referring to FIG. 8A, a minimum distance between the gate electrode 425and the drain electrode 445 is defined as d0, and a horizontal distancefrom the position where the d0 is minimum to the bottom edge of thecontact hole is defined as d1. Further, the sum of the thickness d22 ofthe interlayer insulating layer 430 and the thickness d21 of the gateinsulating layer 420, which is the thickness of insulating layers, at aposition where the distance d0 between the gate electrode 425 and thedrain electrode 445 is minimum, is defined as d2. Thus, to prevent theinterference between the gate electrode 425 and the drain electrode 445,the gate electrode 425 and the drain electrode 445 have to maintain thedistance d0. Because of this, the contact hole 435 should have a taperangle for maintaining the minimum distance between the gate electrode425 and the drain electrode 445.

As a result, the first taper angle θ41 for maintaining the minimumdistance between the gate electrode 425 and the drain electrode 455 isgiven from a first equation 1 below.tan θ41=d2/d1θ41=tan⁻¹(d2/d1)  (1)

In the OLED having high resolution, d1 is 2 μm based on the design rule,and when the thickness d21 of the gate insulating layer 420 is 0.1 μmand the thickness d22 of the interlayer insulating layer 430 is 0.4 μm,the minimum value of the first taper angle is given from the firstequation.

In other words, tan θ41=0.5 μm/2 μm, θ41=tan⁻¹(0.25)=14°.

Therefore, the first taper angle θ41 in the first exemplary embodimentof the present invention should be in a range of 14° to 60°. Inaddition, to minimize the interference between the gate electrode andthe source and drain electrodes, the gate electrode 425 should have apredetermined fourth taper angle θ44. The fourth taper angle θ44 isformed between the side of the gate electrode and the substrate surface,and should be the same angle as a third taper angle θ43. Thus, thefourth taper angle θ44 should be 60° or less, and preferably, in a rangeof 14° to 45°. The gate electrode 425 is formed to have the fourth taperangle θ44 using a taper-etching method after gate electrode material isdeposited.

The anode 470 should have a predetermined third taper angle θ43 toprevent element defects from occurring at an edge of the anode in thefirst exemplary embodiment of the present invention. The third taperangle θ43 is formed between the side of the anode and the substratesurface, and the anode as a lower electrode is patterned to have thethird taper angle θ43 when anode material is deposited and patterned.The third taper angle θ43 should be 60° or less, and preferably, 45° orless.

The structure of OLED illustrated in FIG. 8B does not correspond exactlyto the structure of the OLED shown in FIG. 7. Instead, FIG. 8B providesa view for explaining a principle of obtaining the minimum taper angleof an anode 470′ in the OLED in accordance with the first exemplaryembodiment of the present invention. The taper angle θ43 for the anode470 in FIG. 7 is calculated using the same principle as illustrated inFIG. 8B.

Referring to FIG. 8B, when an anode 470′ as a lower electrode formed ona passivation layer 450′ has a thickness of d4 and a length differencebetween an upper surface and a lower surface of the anode 470′ havingthe predetermined third taper angle θ43 is d5, the taper angle of thelower electrode is given by the equation 2 below.tan θ43=d4/d5θ43=tan⁻¹(d4/d5)  (2)

For example, when the thickness d4 of the lower electrode is 0.1 μm, andwhen the length difference d5 between the upper surface and the lowersurface of the anode 470′ is 2 μm or less based on the design rule, theminimum value of the third taper angle θ43 is obtained from the equation2.

In other words, tan θ43=0.1 μm/2 μm, and θ43=tan⁻¹(0.05)=2.9°.

Thus, the anode 470′ should have the third taper angle in a range of2.9° to 60° in the first exemplary embodiment of the present invention.

When taper angles of the contact holes and the via hole are reduced tobe 60° or less and the taper angle of the lower electrode is reduced tobe 60° or less as is done in the first exemplary embodiment, dark spotsdo not occur in the emission region as shown in FIG. 14.

Referring to FIG. 9, a buffer layer 505 is formed on an insulatingsubstrate 500, and a semiconductor layer 510 having source and drainregions 511 and 515 is formed on the buffer layer 505. A gate electrode525 is formed on a gate insulating layer 520, and source and drainelectrodes 541 and 545 are formed on an interlayer insulating layer 530,and are connected through contact holes 531 and 535 to the source anddrain regions 511 and 515, respectively. An anode 570 as a lowerelectrode is formed on the interlayer insulating layer 530, and isconnected to one of the source and drain electrodes 541 and 545, forexample, to the drain electrode 545.

The contact holes 531 and 535 should have a first taper angle θ51 in arange of 14° to 60°, and preferably in a range of 14° to 45°. The gateelectrode 525 has the same fourth taper angle θ54 as the contact holes531 and 535, and should have the fourth taper angle θ54 in a range of14° to 60°, and preferably in a range of 14° to 45°. The anode 570should have a third taper angle θ53 in a range of 2.9° to 60°, andpreferably in a range of 2.9° to 45°.

A passivation layer 550 is formed over the entire surface of thesubstrate, and has an opening 575 for exposing a portion of the anode570. An organic emission layer 585 and a cathode 580 are formed on thepassivation layer 550 and the anode 570 of the opening 575. The opening575 formed in the passivation layer 550 should have a second taper angleθ52 of 40° or less. The organic emission layer 585 includes at least onethin layer selected from a hole injecting layer, a hole transportinglayer, an R, G, or B emission layer, a hole blocking layer, an electrontransporting layer, and an electron injecting layer.

Referring to FIG. 10, a buffer layer 605 is formed on an insulatingsubstrate 600, and a semiconductor layer 610 having source and drainregions 611 and 615 is formed on the buffer layer 605. A gate electrode625 is formed on a gate insulating layer 620, and source and drainelectrodes 641 and 645 are formed on an interlayer insulating layer 630,and are connected to the source and drain regions 611 and 615,respectively, through contact holes 631 and 635.

A passivation layer 650 and a planarizing layer 660 are sequentiallyformed over the entire surface of the substrate. An anode 670 is formedon the planarizing layer 660, and is connected through a via hole 655formed in the planarizing layer 660 and the passivation layer 650 to oneof the source and drain electrodes 641 and 645, for example, to thedrain electrode 645. An organic emission layer 685 and a cathode 690 aresequentially formed on the anode 670 and the planarizing layer 660. Theorganic emission layer 680 includes at least one thin layer selectedfrom a hole injecting layer, a hole transporting layer, an R, G, or Bemission layer, a hole blocking layer, an electron transporting layer,and an electron injecting layer.

The via hole 655 should have a second taper angle θ62 in a range of 14°to 60°, and preferably in a range of 14° to 45°. The anode 670 shouldhave a third taper angle θ63 in a range of 2.9° to 60°, and preferablyin a range of 2.9° to 45°.

In the OLED in accordance with the third exemplary embodiment of thepresent invention, since the planarizing layer 660 is formed toplanarize the substrate surface before the anode is formed after thethin film transistor is formed, the contact holes 631 and 635 may beformed by the typical method or formed to allow a first taper angle θ61to be in a range of 14° to 60° as is done in the first and secondexemplary embodiments. In the same manner, the gate electrode 625 may beformed by the typical method or formed to allow a fourth taper angle θ64to be in a range of 14° to 60° as is done in the first and secondexemplary embodiments.

Referring to FIG. 11A, a passivation layer 750 is deposited on aninterlayer insulating layer 730, a gate insulating layer 720 and abuffer layer 705 formed on an insulating substrate 700 where a thin filmtransistor 701 is formed, and a planarizing layer 760 is deposited onthe passivation layer 750. The thin film transistor 701 may be formed bya typical method or formed to allow a gate electrode 725 to have apredetermined fourth taper angle θ74 and to allow contact holes 731 and735 to have a predetermined first taper angle θ71 as is done in thefirst and second exemplary embodiments. The taper angles θ71 and θ74 aresubstantially the same as the taper angles θ61 and θ64, respectively, ofFIG. 10. The passivation layer 750 is an inorganic insulating layer, anda nitride layer may be used for the same. The planarizing layer is anorganic insulating layer, and a BCB layer may be used for the same.

Referring to FIG. 11B, the passivation layer 750 and the planarizinglayer 760 are dry etched at the same time to expose a drain electrode745 among source and drain electrodes 741 and 745 of the thin filmtransistor 701, which leads to form a via hole 755 having a second taperangle θ72 of 60° or less, preferably 40° or less. The source and drainelectrodes 741 and 745 are connected to source and drain regions 711 and715 of a semiconductor layer 710 through the contact holes 731 and 735,respectively.

Next, an anode (not shown in FIG. 11B) connected to the drain electrode745 through the via hole 755, an organic emission layer (not shown inFIG. 11B), and a cathode (not shown in FIG. 11B) are sequentiallyformed.

The second fabrication method for fabricating the OLED illustrated inFIGS. 12A and 12B differs from the first method illustrated in FIGS. 11Aand 11B in that how the via hole is formed. In detail, an inorganicinsulating layer such as a nitride layer is deposited as a passivationlayer 850 on an interlayer insulating layer 830, a gate insulating layer820 and a buffer layer 805 formed on an insulating substrate 800 where athin film transistor 801 is formed, and the passivation layer 850 is dryetched so as to expose a drain electrode 845 among source and drainelectrodes 841 and 845 of the thin film transistor 801 to thereby form afirst via hole 854. A gate electrode 825 has a predetermined fourthtaper angle θ84, and contact holes 831 and 835 have a predeterminedfirst taper angle θ81 as is done in the first and second exemplaryembodiments. The taper angles θ81 and θ84 are substantially the same asthe taper angles θ61 and θ64, respectively, of FIG. 10.

As shown in FIG. 12B, a BCB layer as a planarizing layer is coated onthe substrate and a photoresist layer 853 is then formed on aplanarizing layer 860 to expose the first via hole 854. The planarizinglayer 860 is dry etched using the photosensitive layer 853 as an etchmask to form a second via hole 855. The second via hole has a secondtaper angle θ82 of 60° or less, preferably 40° or less. The source anddrain electrodes 841 and 845 are connected to source and drain regions811 and 815 of a semiconductor layer 810 through the contact holes 831and 835, respectively.

The third fabrication method of FIGS. 13A and 13B differs from thesecond method of FIGS. 12A and 12B in that how the via hole is formedusing a photosensitive organic insulating layer. As shown in FIG. 13A,an inorganic insulating layer such as a nitride layer is deposited as apassivation layer 950 on an interlayer insulating layer 930, a gateinsulating layer 920 and a buffer layer 905 formed on an insulatingsubstrate 900 where a thin film transistor 901 is formed, and thepassivation layer 950 is dry etched so as to expose a drain electrode945 among source and drain electrodes 941 and 945 of the thin filmtransistor 901 to thereby form a first via hole 954. A gate electrode925 has a predetermined fourth taper angle θ94, and contact holes 931and 935 have a predetermined first taper angle θ91 as is done in thefirst and second exemplary embodiments. The taper angles θ91 and θ94 aresubstantially the same as the taper angles θ61 and θ64, respectively, ofFIG. 10.

As shown in FIG. 13B, a photosensitive organic layer as a planarizinglayer 960, for example, a photoreactive BCB layer or photoresist layer,is coated on the passivation layer 950 and is subject to an exposureprocess to thereby form a second via hole 955 for exposing the first viahole 954. The second via hole 955 has a taper angle θ92 of 60° or less,preferably 40° or less. The source and drain electrodes 941 and 945 areconnected to source and drain regions 911 and 915 of a semiconductorlayer 910 through the contact holes 931 and 935, respectively.

The OLED in accordance with the first to third exemplary embodiments ofthe present invention may be applied to a bottom-emitting structurewhere the anode as a lower electrode is formed to be a transmittingelectrode and the cathode as an upper electrode is formed to be areflective electrode. The OLED of the first to third exemplaryembodiments may also be applied to a top-emitting structure where theanode as a lower electrode is formed to be a reflective electrode andthe cathode as an upper electrode is formed to be a transmittingelectrode. Further, the OLED of the first to third exemplary embodimentsmay be applied to a double-side-emitting structure where the anode as alower electrode and the cathode as an upper electrode are formed to betransmitting electrodes.

In addition, while the first to third exemplary embodiments of thepresent invention have been described in reference to an OLED having thetypical structure where an anode, an organic emission layer, and acathode are sequentially formed, they may be also applied to the OLEDhaving the inverted structure where the cathode, the organic emissionlayer, and the anode are sequentially formed. Further, they may beapplied to the OLED in which the organic emission layer is formed usingvarious methods such as a deposition method, an inkjet printing method,or a laser induced thermal imaging method.

In accordance with the present invention, the taper angles of a viahole, a contact hole, and a lower electrode are reduced, which allows toprevent defects of the contact hole and via hole and at the edge of thelower electrode, and to prevent defects of the organic emission layer,so that reliability and yield may be improved.

In addition, the pixel defining layer of the organic thin layer of thepresent invention may not be used in order to prevent element defects,which allows to prevent or reduce the defects due to the use of thepixel defining layer from occurring and also to simplify the process.

While the present invention has been described with reference to certainexemplary embodiments, it is understood that the disclosure has beenmade for the purposes of illustrating the present invention by way ofexamples and is not intended to limit the scope of the invention. Thoseskilled in the art would recognize that a variety of modification andchange can be made without departing from the spirit or scope of thepresent invention described in the claims appended below, andequivalents thereof.

1. A flat panel display, comprising: an insulating substrate; asemiconductor layer having source and drain regions of a thin filmtransistor formed on the insulating substrate; a gate electrode formedabove the semiconductor layer; a first insulating layer formed betweenthe semiconductor layer and the gate electrode; a second insulatinglayer having contact holes for exposing a portion of the source regionand a portion of the drain region; source and drain electrodes connectedrespectively to the source and drain regions through the contact holes;a lower electrode formed on the second insulating layer and connected toone of the source and drain electrodes; a third insulating layer forpassivation and having an opening for exposing a portion of the lowerelectrode; an organic emission layer formed on the lower electrode andthe third insulating layer; and an upper electrode formed on the organicemission layer, wherein the contact holes and the lower electrode aretapered with taper angles of 60° or less with respect to a surface ofthe insulating substrate.
 2. The flat panel as claimed in claim 1,wherein the contact holes have the taper angles in a range of 14° to45°, and the lower electrode has the taper angle in a range of 2.9° to45°.
 3. The flat panel as claimed in claim 1, wherein the lowerelectrode is one of an anode and a cathode, and the upper electrode isthe other one of the anode and the cathode.
 4. The flat panel display asclaimed in claim 1, wherein the organic emission layer includes at leastone thin layer selected from a hole injecting layer, a hole transportinglayer, an emission layer, a hole blocking layer, an electrontransporting layer, and an electron injecting layer, and the organicemission layer is formed by a laser induced thermal imaging method, aninkjet printing method, or a deposition method.
 5. The flat paneldisplay as claimed in claim 1, wherein the lower electrode acts as atransmitting electrode, and the upper electrode acts as any one ofreflective and transmitting electrodes, and light emitted from theorganic emission layer is directed toward the substrate, or directedboth toward the substrate and in an opposite direction away from thesubstrate.
 6. The flat panel display as claimed in claim 1, wherein thegate electrode is tapered with the same taper angle as the contactholes.
 7. The flat panel display as claimed in claim 1, wherein theopening of the third insulating layer is tapered with a taper angle of40° or less with respect to a surface of the insulating substrate.